A probe comprising a single signal terminal and at least one ground terminal is generally provided to a conventional electrical characteristics measurement device used for measuring the electrical characteristics of printed substrate circuits, devices, and other components that handle high frequency signals (see Patent Documents 1 to 3, for example). FIG. 8 is a schematic view of a convention electrical characteristics measurement device. In a conventional electrical characteristics measurement device, one end of a coaxial cable 105 is connected to a measuring instrument 106, and a probe 101 is connected to the other end of the coaxial cable 105, as shown in FIG. 8. The probe comprises a signal terminal 102 in contact with a conductor pattern 109 formed on a printed circuit board or another measurement object 108, and a ground terminal 103 that is in contact with a conductor pattern 110 and is at ground potential. In the probe 101, a resistance element 104 is disposed in the vicinity of the signal terminal 102, and the impedance of the resistance element is constant.
In this type of conventional electrical characteristics measurement device, a coaxial cable having a characteristic impedance of 50Ω and a probe having an input impedance of 50Ω (50-Ω probe) are generally used in order to match the impedance of the measuring instrument 106 when the measuring instrument 106 has an input impedance of 50Ω (is a 50-Ω measuring instrument). However, a 50-Ω probe has a problem in that the input impedance affects the circuit operation of the measurement object, and measurement errors are magnified.
In a high-impedance probe in which the load effect is reduced by increasing the input impedance, the effect on the circuit operation of a measurement object is small in comparison with a 50-Ω probe, but the impedance cannot be matched to that of a 50-Ω measuring instrument. For this reason, a 50-Ω probe can be used when it is necessary to match the impedance of the 50-Ω measuring instrument and the probe, to disconnect the ground terminal 103 and the signal terminal 102 of the distal end of the probe must, to form a short circuit, and to connect a load in order to calibrate the entire measuring device, but a high-impedance probe cannot be used in such cases.
A switchable probe is also used in conventional practice (see Patent Document 4). The probe comprises two circuits having mutually different input impedances, and a switch for switching between the two. In the conventional switchable probe, one of the circuits has an impedance of 50Ω and the other circuit has high impedance. The effect on the circuit operation of a measurement object can be minimized and impedance matching with a 50Ω measuring instrument can be ensured by switching the impedance during calibration and during measurement by using a switch.
[Patent Document 1] Japanese Laid-open Patent Application No. 4-206845
[Patent Document 2] Japanese Laid-open Patent Application No. 2001-133482
[Patent Document 3] Japanese Laid-open Patent Application No. 58-90176
[Patent Document 4] Japanese Laid-open Utility Model Application No. 2-32064